They really can't seem to catch a break, can they? First came the problem with unintended acceleration, originally attributed to sticking floor mats. Then came problems with their hybrid powertrains, and their deceleration failure over rough terrain. And finally the steering issues with the Corolla, the world's highest selling car model. Things have gotten so bad, they've been forced to appear before Congress, to repent for their sins. Those sins appear to be multiple in nature, not just manufacturing but political and marketing. We'll stick to the design and manufacturing ones here.
First off, I'm not an automotive engineer. I'm an electronic engineer, with a strong background in control systems and software. While this leaves me short in some avenues it gives me insight and experience in problems experienced in non-Automotive applications with similar issues. And being a serious car enthusiast also gives me plenty of background information and history, hopefully enough to prevent me from making a fool of myself.
When it all boils down, a working car does three things- it moves, it stops, it steers. Everything else is just icing on the cake. Up until a few years ago, all of these systems were handled by for all practical purposes centuries old mechanical means. A cable attached to a carburetor or fuel injection system controls a throttle or a valve allowing for the fuel and air to be introduced into the cylinders. (This is a gross simplification, but enough for our purposes.) As you press down on the accelerator, a return spring is compressed, applying feedback pressure to your right foot.
In the case of the brakes, a cable pulled two brake shoes towards a rotating drum applying friction, or piston was sent into a pressurized cylinder, applying force on another cylinder which pushed fluid into pistons, causing them to expand and apply force against a rotating disk. Again a mechanical action caused another mechanism to directly apply a force to an object. The feedback system is as you increase pressure on the brake piston, the fluid inside gets compressed and pushes back against the pedal.
With steering (which I won't get into heavily) you turned a gear which pushed a special lever called a "rack" (or sent a course of ball-bearings) moving the front wheels' relative position towards one direction or the other. The feedback involved is a bit more complex- as a car moves faster the wheels are being pushed towards the so-called "path of least resistance". There is less friction when the car moves straight, so the car "wants" to move straight. That is because when it moves straight all four wheels are moving in the same direction, at the same speed, but when it is turning, the front and rear wheels on the inside of the turn is moving slightly slower than those wheels on the outside of the turn. The more the wheels are turned away from a straight line, the more they "push" against the steering wheel. This generates a weak form of negative feedback which tries to set the car straight again.
In some modern cars, it can get quite a bit more tricky. Gone are purely mechanical systems; now we have "drive by wire". Whenever you push on a pedal, turn a wheel or activate a switch a device has to read the linear or circular position of that control, report that position to a processing unit, and that in turn tells electromechanical actuators what to do.
So when you step on the gas, a potentiometer (variable resistor) or an encoded switch (which reads a set of "1"s and "0"s either printed on a media or as a conductor pasted between two electrodes) tells something else what the position of the gas pedal is. It is up to this device, working alongside the engine management software, that tells the car how much gas to give. The brakes act almost the same way, except they decide how much pressure to apply on the brake pads. Drum brakes are very rare in modern cars, even more rare in cars with anti-lock brakes. They used to make more ABS cars with rear drum brakes, but only the real cheap ones now have that. Drive-by-wire steering is not very common, mostly used by really expensive luxury cars to create variable-rate steering.
Encoders can get quite complicated. While they use binary they use it in a fancy way known as "Gray code." In Gray code when you advance forwards or backwards from one number to another, only one bit moves at a time. For example, in 4 bit binary systems the number 8 is expressed as "0100" and the number 7 is "0111" so when you jump from 7 to 8 all four bits change. But in Gray code, 7 is "0101" and 8 is "0100. In a complex encoder. say 10 bits the printing resolution or the resolution of the smallest switch is actually 1 bit less, so instead of the smallest physical change being 2^10 (1/1024) the size of the total system, it is 2^9, or 1/512th the size. This makes it a lot easier to manufacture encoders, rotary or linear position sensors, etc.
Steering and brakes do have to have a failsafe system. Hydraulic brakes need to work even if the power steering pump fails, and steering has to work as well, even if very badly. Many cars are so hard to steer when the pump fails or if the car stalls that they are for all practical purposes unguided missiles. So just because something "has" to be a certain way, there is no guaranty that it will be.
Toyota's acceleration problem has been attributed to "stuck floor mats" and worn out accelerator sensor bushings. This actually makes sense, in that these are indeed ways that a mechanical object can prevent the return mechanism from working the way it should. The pedal assemblies made by Denso (if you've ever owned a Japanese car, half the parts were made by them) work fine, their North American supplier has a problem with the return spring mechanism getting stuck.
But there has been a debate over whether the problem is really mechanical in nature- or is it software?
The problem with the Prius and several species of Lexus hybrids are definitely software and application-sensitive in nature. In a hybrid, when you step on the brakes (or in some European models the minute you step off the gas) the regenerative braking system tries to recapture the energy used in accelerating the car to recharge the batteries. Instead of having a piston press up against a disc the wheel motors become generators, sending energy back into the battery. But when things get a bit crazy, the car decides to slam on the pads.
So how does the car decide what is nominal driving, and what is a "panic stop"? Well that has to play with a combination of factors, dealing with pedal position, speed, and acceleration (in this case acceleration means slowing the car down.) It isn't easy to write algorithms to decide when to kick in the hydraulics, partly because it hurts the cars efficiency. A balance has to be struck between safety and economy.
The complaints I have heard seem to come from drivers having problems in bad pavement, or "washboard" conditions. It is possible that the wheels are sensing a repetitive pattern of increasing resistance, and are mistaking that for wheel lock on one or more wheels. The ABS system rather than making sure that all four wheels are going straight, misapplies reduced pressure on each wheel, preventing the car from slowing down.
So my experience and "intuition" tell me this isn't an electronic problem, and it probably isn't a supplier problem (as Toyota is saying) but it is a problem with trying to cut costs and use the same basic assembly across the entire company platform. Normally this is a way smaller car companies cut costs- obviously if you want to make your limited-volume vehicles cheaper, steal parts from your other vehicles' parts bins. The problem here is when you do this with bigger companies, you tend to make those parts to suit the lower cost end of the chain, not the higher. Why put a LS or a GS part in a Yaris, when you can put a Yaris part in a Lexus! Sure you save money, often tons of it, but your customers who pay $85,000 for a car get the same part the guy who paid $13,000. It does end up hurting your reputation, as it is clearly doing here.
I think the real reason why they are going after Toyota so hard is their previous bullet-proof reputation. While a lot of it is American chauvinism, I've seen many of the "excuses" why the Japanese do not import American cars, American beef, and come to think of it so many American items and the reasons are most often xenophobic in nature! We don't seem clean enough, or our cows aren't treated as well, or our apples have too many chemicals on them.
Before people get in a tizzy, one reason I've heard Japanese automotive industry apologists claim why their cars break down in the US is because Americans eat in their cars, drink in their cars, and wear regular shoes in their cars, while Japanese would never dream of doing this. If that isn't a form of nationalistic superiority, or even racism, then tell me why do they make cupholders in cars designed for the Japanese market! But then again I'll never forget why the head of marketing for Peugeot explained why the 405 Mi16 wasn't originally imported into the US. His response was, "you don't serve fine strawberries to pigs." So it appears that anti-Americanism is rampant throughout our industrial allies. And that Peugeot quote came from around 1988, a long time before our Gulf-war era tensions.
Now one of the suggested cures is a lockout device which will prevent the engine from accelerating when the brake pedal is engaged. This would ruin the driving experience of anyone who has ever tried to launch a car, or anyone who two-foot drives (using both feet simultaneously on an automatic transmission vehicle. For some high speed maneuvers, especially on snow or mud or loose gravel drivers get improved handling using this. I sometimes switch to two-footed driving in very heavy traffic just to give my right foot a break (but only when I'm using my automatic, not my two manual transmission cars. You really don't need to keep your foot on the brake with a manual, but it confuses the guy behind you who thinks you've just stepped off the brakes and onto the gas.) There are really good drivers who can use a method of pressing on both simultaneously with one foot called "heel-and-toe" driving. This is used when you want to downshift a manual transmission car, and you increase the engine speed while on the clutch so that it matches the speed the engine will need to go when you change gears up. This is often called "rev-matching" and is actually done for you on the new Nissan 370Z. Amazingly, this is the first car ever with this feature! If the cure is essentially castrating the car, count me out.
When you consider how ubiquitous and important the car is to our everyday lives, combined with the variety and complexity of many modern cars today, it is rather reassuring that so few of these seriously dangerous recall conditions exist. If even the worst estimates are true, and this is responsible for a few hundred deaths worldwide in a fleet of several tens of millions of vehicles driven billions of miles a year, this ends up being a rather safe form of transportation. Toyota will try their best to rehabilitate their image, but I think this will in the long run cost them some market share. VW really wants to take over the #1 spot worldwide, GM wants it back, and Ford has really improved their fleet in the last few years. We are seeing more competition, and every manufacturer will have to work very hard to squeeze out an advantage.